A dolphin was swimming 28 meters below the ocean's surface. It changed its depth to avoid a predator and ended up 21 meters below the surface. What was the change in the dolphin's depth?

7 meters

Step-by-step explanation:

Now write an equation that represents the situation. You want to find the change in the dolphin's depth, so subtract its beginning depth from its final depth.

he used the algebra tiles correctly.

step-by-step explanation:

given : 3x(2x-1)

solution :

since he used algebra tiles .

armando's solution is the same in the solution shown in figure

as he distributed 2x into x and x and wrote them vertically

and also write   -1 vertically

and 3x into x and x and x and wrote it horizontally

performing the multiplication in the algebra tiles we get

and solving the given expression :

3x(2x-1)

since solving the expression using tiles   and solving the expression algebraically we get the same answer

thus   we can choose the first statement as the answer to this problem which is "he used the algebra tiles correctly".